1. Field of the Invention
This invention relates generally to methods for the recovery of metal values from sulfide ores by reduction and more particularly, to the use of scavengers to combine with the sulfur in conducting the reduction procedures and reducing air pollution.
2. Description of the Prior Art
The metal industries are finding it very difficult to meet the air pollution regulations and standards promulgated by federal and state agencies within the last few years. The use of conventional pyrometallurgical processes in the production of metals from sulfide ore concentrates results in the emission of suspended particulate matter and sulfur oxides. Of these air contaminants, it has been found that sulfur oxides are much more difficult to control. Sulfur oxide emissions result from the smelting of sulfur-bearing materials. For example, copper exists in various other forms in nature, such as native copper and copper oxides, carbonates and silicates, but the primary sources of copper exist as low-grade deposits of copper sulfide ores in which the principal copper mineral is chalocopyrite, and which in most instances also contain some iron sulfide.
In an effort to comply with sulfur oxide air pollution regulations, the metal companies have initiated research programs to develop methods for recovering the sulfur oxides being emitted from their smelters. So far, however, no economical method for reducing sulfur oxide emissions to acceptable levels has been reported.
While the metal industry is making an all-out effort to develop an economical method to control sulfur oxide emissions so as to comply with present regulations and standards, there is an ever-increasing demand from the public for further improvement in the quality of the nation's atmosphere. This is resulting in the promulgation of even more restrictive regulations and standards. The continuation of this trend of more restrictive standards may eventually result in the metal producers finding it economically unfeasible, if not technically impossible, to comply with these standards.
Most proposed methods for controlling sulfur dioxide, the major sulfur oxide contaminant, contemplate the conversion of sulfur dioxide to sulfuric acid. However, even if an economically feasible method is found for converting substantially all the sulfur dioxide to sulfuric acid, the disposal of large amounts of sulfuric acid presents another pollution problem.
In their combined state, metals such as copper, nickel and iron are usually chemically combined in the ores with non-metallic elements such as sulfur and oxygen. In winning these latter metals from their ores it usually is necessary to perform a reduction operation where the anions or non-metallic elements such as oxygen, sulfur, etc. which have a negative charge, are separated from the cation metal constituent which has a positive charge. In the reduction process, the metal is separated from the oxygen or sulfur by virtue of the combination of the oxygen or sulfur with reducing gas and by virtue of the diffusion or migration of the metal ions. The rate at which reducing gas combines with the non-metallic elements, sulfur and oxygen, can be controlled by controlling the pressure, temperature, rate of the flow of reducing gas, composition of the local gas phase and surface area of the solid.
Of particular interest in processes of this type is the recovery of metals such as nickel and copper which are extremely valuable. The United States leads the free world in nickel consumption, about 30%, while producing less than 3.3% of the free world nickel output. This imbalance provides enouh impetus to explore the possibilities of obtaining nickel from the various sulfide ores such as Duluth Gabbro, the largest nickel sulfide ore body in the United States, and copper from ores such as chalcopyrite and the like.
The specific present day procedure for extracting nickel and copper from sulfide ores involves an initial roasting operation in which the sulfur level of the concentrate is adjusted to an optimum level for smelting operation. In the smelting operation, a copper-nickel-iron matte is produced, which matte is then blown to white metal in a converter by selective oxidation of iron sulfide and its removal in the form of a slag. Thereafter, the white metal is annealed over a period of several days to promote differential crystallization. This results in a separation of copper and nickel sulfides. The nickel sulfide and copper sulfide are then separated by flotation and the nickel sulfide product is processed further by roasting or electrolytic refining and marketed as NiO or Ni.
The present day procedure for extracting copper from sulfide ores involves preliminary roasting for partial elimination of sulfur contained in the copper concentrate followed by smelting in a reverberatory furnace to concentrate copper into matte, followed by conversion or Bessemerization of matte to blister copper which is then cast into anodes for electrolytic refining.
These procedures involve many steps and a substantial possibility is inherent in these processes for pollution of the atmosphere by SO.sub.2. Under the present environmental restrictions and regulations, it is highly desirable that there be provided procedures which will simplify the operations by which nickel, copper and other metals are recovered from sulfide slags and also to provide a process which will not pollute the atmosphere in its operation.